A PDA was the machine people carried before smartphones did the same jobs. It was small enough for a pocket, and it held the things you needed to look up during the day: numbers, addresses, a diary. Later ones could send email and go online, which is the point where the phone took over. The source marks this use dated, and that fits what you see now: it turns up in writing about the 1990s and 2000s, in museum labels and in company records, more than in talk about today's devices. If you meet PDA in a modern conversation between friends, it is far more likely to be the other meaning.

PDA is what a couple does in front of other people: holding hands on the bus, a hug at the station, a kiss in a cafe. The word is nearly always about how much is too much, so it turns up in sentences about rules, limits and embarrassment. It is informal, and it works both as a countable noun (that was a lot of PDA) and as an uncountable one (she is not into PDA). It is also slightly judging by nature: people rarely say PDA about their own happy moment, and often say it about someone else's. Because it names a whole action, it does not take a verb like kiss after it; you engage in PDA, or you just say there was a lot of PDA.